1. The Field of the Invention
The present invention relates generally to supported catalysts and methods for making and using such catalysts (e.g., in the direct synthesis of hydrogen peroxide). More particularly, the present invention relates to the manufacture of supported catalyst using a porous support and a nanocatalyst solution.
2. The Related Technology
Transition metal (e.g., noble metal) catalysts play a very important role in numerous industrial chemical processes, including pharmaceuticals manufacturing, petroleum refining, and chemical synthesis, among others. Cost pressures and the need for improved synthesis routes have led to continued improvement in catalyst performance. For example, improvements in catalysts for the direct synthesis of hydrogen peroxide using oxygen and hydrogen have led to direct synthesis routes that are now economically feasible compared to the traditional synthesis routes using alkylanthroquinones.
Transition metal catalysts are typically small metal particles or crystallites supported on a support material. Since catalyst performance generally increases with decreased particle size, great efforts have been made to obtain catalysts with very small particle sizes. Recently, particle sizes of less than 10 nm have been achieved for some catalysts.
The catalyst particles are typically deposited on a catalyst support material. Catalyst supports are often porous materials with high surface areas. The high porosity and high surface area are needed to allow a sufficient number of catalyst particles to be loaded on the support material. The type of support material used and the characteristics of the support material often have a significant impact on catalytic performance. The selection of the support material can beneficially enable or undesirably limit the type of reactor or conditions in which the catalyst can be used. For example, to use a catalyst in some reactors the size of the catalyst can be important.
Another important performance characteristic of particle catalysts is selectivity. Many particle catalysts are inherently capable of catalyzing more than one reaction for a given reaction mixture. In many cases, the different reactions are catalyzed by distinct active sites on the catalyst particle. Catalysis is achieved as reactants bond with catalyst atoms at the surface of the particle. The arrangement of the exposed atoms may determine catalytic properties of the catalyst. While one crystal face exposure may catalyze a desired reaction, another crystal face exposure may catalyze an undesired reaction.
Recently, manufacturing techniques have been developed that allow catalysts particles to be formed with a controlled crystal face exposure. Examples of supported nanocatalysts are disclosed in U.S. Pat. Nos. 7,045,479 and 7,011,807. The supported catalyst particles can be manufactured using an organic control agent. The control agent molecules are reacted with catalyst atoms in solution to form organometallic complexes. The complexed atoms are then allowed or caused to form a colloidal suspension and subsequently nanoparticles. As the nanoparticles form, the control agent molecules influence the crystal face exposure. Particles formed using this method have shown dramatic improvements in selectivity, reduced particle size, and improved particle stability.
While these recent improvements in catalyst performance have been substantial, there is still a need for improved catalyst synthesis routes and improved catalyst performance. However, it is often very difficult to know what feature of the catalyst or reaction system can or should be improved. Many in the art have focused on reducing catalyst particle size because it is relatively easy to predict an improvement based on reduced particle size. There are, however, many factors that can affect the actual performance of a catalyst in a reactor system.